The Tusa, Diverite, and Cressi are all Seiko computers. Aeris, sherwood, genesis, oceanic are all Palegic computers.
Each company will have slight differences, for example in the Seiko computers, Tusa, selling the exact same computer as Diverite, offers a free scratch protector, and already has the down load cable available, and has a 2 year warranty. Diverite MSRP is actually a few dollars more and has a 1 year warranty.
The Cressi is the same, except can only be used for 1 gas instead of 2.
Really your best bet to find a good deal, is contact an authorized dealer (make sure you get warranty on computers... it's the one thing that will have to go back if there's a problem... unlike a reg where you can just change a part out...).
You should be able to take your pick between them and get out for under $300..